Boat Hull Speed Calculator
Estimate a displacement boat’s hull speed from its waterline length — the natural speed limit set by its own bow wave.
How it works
Hull speed (kt) = ratio × √(waterline length in ft) · classic ratio = 1.34
A displacement boat pushes a bow wave whose wavelength grows with speed. Near a speed–length ratio of 1.34, that wavelength matches the waterline length and the boat sits in the trough of its own wave, so climbing further takes disproportionately more power — the traditional "hull speed" limit. Light or fine-hulled boats can exceed it and semi-displacement or planing hulls break past it entirely, which is why the ratio is adjustable rather than a hard wall.
Worked example
For a 30 ft waterline at the classic 1.34 ratio: hull speed = 1.34 × √30 = 1.34 × 5.48 ≈ 7.34 kt, about 8.4 mph. Doubling the waterline to 120 ft would only double the hull speed, since it scales with the square root of length.
Sources: Hull speed — Wikipedia
Frequently asked questions
Is hull speed an absolute limit?
No — it is the speed at which wave-making drag rises steeply, not a wall. Narrow, light or long hulls routinely exceed a 1.34 ratio, and planing hulls climb onto their own bow wave and leave the concept behind entirely.
Why does waterline length matter more than overall length?
The bow wave is set by the length of hull actually in the water. Overhangs above the waterline do not lengthen the wave, so two boats with the same overall length but different waterlines have different hull speeds.
What ratio should I enter?
Use 1.34 for a classic full-displacement estimate. Fine-hulled cruisers may reach 1.4–1.5, and semi-displacement powerboats push well past 2 before planing, so raise the ratio to match your hull type.
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This calculator is for educational purposes. Double-check important results before acting on them.